linux-kernel - Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI

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Date:   Thu, 23 Mar 2023 16:25:25 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
        openrisc@...ts.librecores.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        x86@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Nicholas Piggin <npiggin@...il.com>,
        Guo Ren <guoren@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI

On 22/03/23 15:04, Peter Zijlstra wrote:
> @@ -798,14 +794,20 @@ static void smp_call_function_many_cond(
>  		}
>  
>  		/*
> +		 * Trace each smp_function_call_*() as an IPI, actual IPIs
> +		 * will be traced with func==generic_smp_call_function_single_ipi().
> +		 */
> +		trace_ipi_send_cpumask(cfd->cpumask_ipi, _RET_IP_, func);

I just got a trace pointing out this can emit an event even though no IPI
is sent if e.g. the cond_func predicate filters all CPUs in the argument
mask:

  ipi_send_cpumask:     cpumask= callsite=on_each_cpu_cond_mask+0x3c callback=flush_tlb_func+0x0

Maybe something like so on top?

---
diff --git a/kernel/smp.c b/kernel/smp.c
index ba5478814e677..1dc452017d000 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -791,6 +791,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 			}
 		}
 
+		if (!nr_cpus)
+			goto local;
 		/*
 		 * Trace each smp_function_call_*() as an IPI, actual IPIs
 		 * will be traced with func==generic_smp_call_function_single_ipi().
@@ -804,10 +806,10 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		 */
 		if (nr_cpus == 1)
 			send_call_function_single_ipi(last_cpu);
-		else if (likely(nr_cpus > 1))
+		else
 			send_call_function_ipi_mask(cfd->cpumask_ipi);
 	}
-
+local:
 	if (run_local && (!cond_func || cond_func(this_cpu, info))) {
 		unsigned long flags;